The planet is in the middle of what astronomers call "the Goldilocks zone", that hard to find place that is not too hot, not too cold, therefore where water, which is essential for life, does not freeze or boil.
It also has a mild surface temperature of near 22 degrees Celsius/72 degrees Fahrenheit, scientists say.
"NASA has this space telescope called Kepler, which has been staring for the past couple of years at one particular area of sky containing over 100,000 stars and it's been looking for very slight dips in the star's brightness as something goes across in front of the star," Ian Ridpath, from the Royal Astronomical Society, told Britain's Sky News.
"Now, if it does that three times in succession, then the NASA scientists think that is good enough to conclude that what's causing the dip in light is a planet," he added.
The new planet's existence was confirmed on Monday by NASA, along with other discoveries made by Kepler, which was launched on a planet-hunting mission in 2009.
It is the first planet within the habitable zone that has been confirmed by the telescope, which has already found Earth-like rocky planets elsewhere.
Twice before, astronomers have announced a planet found in that zone, but neither has been as promising.
The new planet, named Kepler-22b, has key features it shares with Earth.
It circles a star that could be the twin of Earth's sun, and at approximately the same distance.
The planet's year of 290 days is also close to Earth's.
It probably has water and rock, but is a little large for life to exist on the surface.
The planet is about 2.4 times the size of Earth and could be more like the gas-and-liquid Neptune with only a rocky core and mostly ocean.
Kepler cannot identify signs of life, only where the conditions might be right for it to thrive.
The latest discovery has been confirmed in several ways, including by two other telescopes.
The planet is 600 light years away. Each light year is 5.9 trillion miles (9.5 trillion kilometres).
It would take a space shuttle about 22 million years to get there.